Difference in measurement of a diameter of a Shaft between micrometer and CMM

[[Ma...]]

Good morning everybody, 

I am having a big dillemma in measuring this diameter of 10, As I have differences of 5 microns on all the diameters, measuring between micrometer and Zeiss. 

 

I have used various micrometer to verify and they all give me 5 microns less than the Zeiss. 

I have also recalibrated CMM, use temperature sensors, everything but, the difference still remains. 

I used gauge of 11 to measure with the same program, and they give me equal, with the same evaluation method. 

 

Can anyone help me on this? 

 

 

 

 

[[Ma...]]

Are you using LP for reporting diameter value? Using filters and so on?

Also used machine and head can have impact too - but i don't work in those tolerances so for me is 5 microns ok ( it's 0,005mm )

[[Ma...]]

I have followed the cookbook for the evaluation and also the number of points and scanning speed. 

[[Ri...]]

The answer is probably somewhere in the form of the feature or something with filters. 

Having said that, if you are chasing microns, I wouldn't recommend holding either the part or the micrometer in your hand - the heat transfer from your body to your part/mic will cause you issues. 

[[Da...]]

Can you ask for roundness on the cmm? chart the results and post them here please. Also can you use something to verify that the Mic is correct? Since the Mic reads lower, check to see that the stationary anvils surface isn't worn.

[[Ra...]]

Are you doing a 2-point diameter characteristic? Or a regular diameter?

Do you have a calibrated artifact, like a ring gauge, to compare?

[[Ow...]]

I've seen this discrepancy a thousand times over the years.

I'm betting it is a pressure or hand generated heat transfer like Richard Shomaker mentioned.

Are you using the friction thimble or the bigger body to set your zero on the micrometer standard?

When using an annually calibrated or newly certified micrometer standard, that's kept in a climate controlled room, you should always use the friction thimble when setting zero on the micrometer. This ensures that you are applying the same exact force between the standard and the test piece.

Also as mentioned, if the part is out of round, it will measure differently and you should be able to see that with the micrometer when rotating and checking the part.

When measuring micron's, it's also good to use the same shape standard (a certified xxx cylinder) instead of the flats on a regular micrometer standard because that will ensure you are measuring the same amount of surface and force as the cylinder you are checking. 

 

Edited September 16

[[Ma...]]

The roundness, I have is 0.001.

Yes, I used micrometer to measure a gauge of 11mm and it was perfect, 11.000 and also in the CMM, the 11mm gave me 11.000. Then, I checked the cylindricity and it was also 2 microns. 

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[[Da...]]

Sorry, but I can't view the image

[[Ma...]]

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[[Ma...]]

This is a common issue when you try and measure a diameter against a two-point gage. What is the evaluation method set to? LSQ or Minimum circumscribed? Are you comparing the measured point that are directly across from each other?

Just remember you are using a $200.00 tool with a fixed measuring method, 2-point distance, to check a CMM with different algorithm settings.

[[Wo...]]

How are you holding the part? If you have it vertically in a chuck - the higher you measure the more likely it is that the part is moving imperceptibly. I run into this all the time. Also, I don't really trust QuantuMike - the spindle moves too fast and it's easy to apply to much pressure. Regular digimatic or High precision mics work much better.

[[Ch...]]

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I vote this as best answer. I get this all the time.

There are a miriad of factors as to why there could be difference.

 

usually the outcome is : we dont trust the CMM , it cant measure diameters, so mask the size on report but still report position 😅

Edited September 17

[[Da...]]

A scanning sensor is all about compromise. The software has to find the best calibration for different speeds, different probing forces, different deflections of the probe. After calibration it all comes down to one calibration setting combined with a deflection dependent correction factor.

You will see the problem this way: take a known gauge plug and a gauge ring, both calibrated by a good lab (ideally the SAME lab), calibrate a probe and scan them both. You WILL see a difference in the precision of the scan. If you need better values, you need to optimize your calibration values and saving them as separate probes, maybe a diameter 5 probe for inner diameters and one for outer diameters, and if in special cases you do the optimization for different diameter ranges and different speeds. For most other purposed you can continue using the standard calibration.

This will yield much better results that are directly comparable to simpler methods like micrometers.

[[Fl...]]

Hello, 

I always find a difference of 0.002mm to 0.003mm to the CMM.

The reason is explain below : there is a little deformation on part and micrometer.

[[Te...]]

make sure the diameter characteristic is using Minimum Circumscribed Element

[[Lu...]]

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Hi

My experience is that simpler measurement way gives more accurate results, i. e. mike wins with CMM.

 

 

I assume the 9.994 diameter is a cylinder of more or less constant diameter and no matter the spot you measure the results is more or less similar

 

Whats a "gauge of 11"?

Did you try to measure a 10mm gage block with CMM and with mike?

I'd set the mike to zero and check it at 10mm with a gage block; not much probable to find the culprit here, but worth to try as it doesn't cost much

Do you use RDS/CAA and qualify styli in twelve angles only? Then the accuracy is bit worse

I'd check the qualification parameters

I'd check the two point measurement

I'd check the diameter with different styli/stylus systems